A conventional cellular mobile/radio system is composed of numerous mobile units (MUs), such as handsets operated by individual users, which home-on associated base stations or base units (BUs). A single base station serves a number of mobile units that lie within a simply connected geographical area--a cell--identified to that single base unit.
Most of the current cellular systems use a registration method to keep track of the locations of the mobile units. A specific database serves as a controller for the units within an assigned cell. Each mobile unit is required to inform, that is, register with the base station, its location when the user moves from the original cell into a new cell. This registration process is typically accomplished automatically by the mobile unit, but the registration may also be effected manually by the user of the mobile unit. The base station will, in turn, pass the registration to the serving switching office.
Since a conventional cellular mobile system generally covers a geographical area larger than a single switching office, the registration process requires the coordination among a plurality of switching offices, and such registration process may involve the use of centralized databases in the serving area to keep track of mobile locations.
When multiple switching offices are involved, the handoff process to maintain an established call as the user moves from one cell (original) to another cell (secondary), becomes somewhat complicated. To ensure a smooth and interrupt-free handoff, two parallel communication paths must be set-up between the mobile unit and original and secondary base stations serving the original cell and secondary cell, respectively. These two communication paths must be merged in the associated switching offices until the original base station breaks its communication path with the mobile unit. Current cellular mobile systems use complex and cumbersome procedures to handle such handoffs; in practice, dedicated signaling and switching procedures are required to handle such handoffs in order to allow movement from one cell to another, that is, one base station to another when the base stations are connected to different switching offices.
Cellular systems separate potentially interfering systems by assigning each MU to a specific BU. Co-channel interference is avoided by not assigning the same channel frequencies in contiguous cells. The cellular approach thus discards co-channel interference information even though it might be used to increase the capacity of the system or improve system performance. Potential interference is information about a signal that might be exploited by suitable signal processing.
The approach in accordance with the present inventive subject matter discloses how MUs may, most effectively, interact with more than one BU using a generalized diversity technique. The disclosed approach represents a substantive improvement to prior art diversity techniques. Representative of one conventional approach is the material on optimal combining as set forth in the articles "Optimum Combining in Digital Mobile Radio with Co-channel Interference," IEEE Journal on Selected Areas in Communications, vol. SAC-2, pp 528-539, July 1984, and "Optimum Combining in Digital Mobile Radio with Co-channel Interference," IEEE Transactions on Vehicular Technology, vol. VT-33, pp 144-155, August 1984, both by J. H. Winters. The present generalized approach also is an enhancement to another conventional avenue of approach related to resource sharing that is discussed by A. S. Acampora and J. H. Winters in the publication "A Wireless Network for Wide-band Indoor Communications," IEEE Journal of Selected Areas in Communications, vol. SAC-5, no. 5, pp 796-805, June 1987. Another conventional avenue of approach is "macro-diversity," which uses geographically dispersed antennas; the paper entitled "Macroscopic Diversity in Frequency Reuse Radio Systems," IEEE Journal on Selected Areas in Communications, vol. SAC-5, no. 5, pp 862-870, June 1987, published by R. Bernhardt, is exemplary of this procedure. In Bernhardt's paper, a downstream transmitter provides diversity that can be exploited to optimize transmission performance while controlling interference to other MUs. This is done by pre-conditioning the signals transmitted to the several MUs in a centralized processing system that performs all signal pre-processing, switching, control, signaling, location database updates and maintenance functions. This centralized processor may be referred to as the Signal Processing and Switching Center (SPSC); the SPSC might be co-located with a telephone switching center. As alluded to above, however, such an approach can be complicated and cumbersome.
In addition to the advantages accruing from more sophisticated signal processing, using several BUs to serve each MU also provides advantages that normally accrue from diversity, that is, using several different channels for transmission provides a greater statistical robustness to fading than a single channel can provide. This effect can also be obtained in other ways such as frequency diversity in spread spectrum signaling, as described by J. Holtzman, "Rayleigh Fading Effect Reduction with Wide-band DS/CDMA Signals," Proceedings IEEE Globecom '91 Conference, paper 16.7, or by using time diversity in coding or time-spreading, as described by M. L. Honig, et al., "Spread-time Code Division Multiple Access," Proc. IEEE Globecom '91 Conference, paper 25A.1. However, these systems consume measurable channel capacity to achieve diversity.
One of the constraints with the cellular approach is that the capacity of the system is bounded once the available bandwidth has been fixed. One way to increase this capacity is to reduce cell sizes and, concomitantly, transmitted power so that the generic cellular architecture can be reapplied but on a somewhat smaller (microcell) scale. This implies that the same engineering, system planning and coordination between micro-cells must be conducted as in the original cellular system, but on a more complex scale. Furthermore, the reduction of the distance to cell boundaries leads to more microcell boundary crossings with concomitant increases in switching, signaling, and location database activities related to increased handoff rates.
Many of the foregoing shortcomings and limitations were addressed in a paper entitled "A Generic Model for Tetherless Access to the Telephone Network," authored by J. W. Lechleider (the present inventor) and published in the Proc. IEEE Globecom '92 Conference. In this paper, generic models of radio access between base units and mobile units were developed and used to determine optimum combinations of transmitter and receiver vectors for transmission from the base units to the mobile units (downstream) as well as transmission from the mobile units to the base units (upstream). The optimum vectors were determined under the presumption that the receiver was arranged with preliminary equalization to eliminate interference between different vectors in the same sequence of data emitted by a given base unit (so-called vector intersymbol interference). With this presumption, it was only necessary to optimize the "one-shot" transmission path or channel that results from transmitting a single vector. Moreover, the criterion used for optimization was to maximize the received signal to mean-square error ratio at the signal receiver portion of the mobile unit. Because the maximum value could be made as large as desired merely by increasing the transmitted power, the transmitted power is constrained in the optimization procedure. This optimization procedure, however, fails to take into account interference that might be caused at the receivers of other mobile units within the range of the base units used to transmit the signal vector to the given mobile unit. Any transmission schema, to be especially effective in more practical applications, must consider the interference cause in other mobile units.
A relatively new technology closely related to the cellular mobile systems is PCS--Portable Communication System; in this technology field, base units communicate with mobile units operated by PCS users or "rovers". One solution proposed to accommodate a high density of PCS users/rovers is to arrange microcells in substantially the same manner that they are used in mobile radio systems. Thus, each base unit has a geographical area assigned to it in the sense that all PCS users in the geographical area would be served by the base unit in that area. There are many concerns with such an approach, not the least of which is the level of switching activity and location database updates that would be required. In addition, it is anticipated that frequency re-use might be more constrained than it is in mobile radio systems. Based on the teachings of the prior art in the field of cellular mobile systems, an alternative to the one base unit per cell arrangement might be to use multiple base units in a cell that is somewhat larger than the anticipated microcells and to use diversity transmission techniques downstream and diversity reception upstream. Linear diversity systems might thus be employed to replace the space division multiplexing in microcell systems with what might be called diversity division multiplexing.
By way of terminology in what follows, the term base unit is used as a universal descriptor to cover both the PCS and the cellular environments, whereas the term mobile unit is used as universal descriptor to cover both the PCS and cellular environments. In general, a BU includes a transmitter for downstream transmission and a receiver for upstream reception. Also, a MU also includes a receiver for downstream reception and a transmitter for upstream transmission. The description sets forth the context of the transmission, that is, whether downstream or upstream, so the particular transmitter or the particular receiver being considered will be clear from the context.